The invention relates to apparatus for filtering molten plastics and for extruding plastics strands, more particularly for the production of plastics granulate, with a filter which is exchangeable by movement transverse to the stream of plastics and with a die head from which the plastics strands are discharged.
The production and processing of plastics frequently calls for filtration of the product. Known filter systems give rise to substantial difficulties which are due to the fact that changing the filter causes dirt to be returned into the product path while temperature-sensitive plastics may suffer from damage which ranges from discoloration to carbonisation. There are various reasons for thermal damage. In known devices the distances between the filter and the die head are relatively long so that the plastics material comes into contact with relatively large metal surface areas, even downstream of the filter, such metal surfaces having high temperatures since the entire device must naturally be heated. In order to minimise the size of such hot metal surfaces the diameter of pipes and ports through which molten plastics folows in known devices have been minimised but in the region of the filter whose surface area is made relatively large in the interests of a high throughput, this leads to the creation of dead zones in transition regions between small and large cross-sections of the heater device, the plastics material being also thermally damaged due to excessive dwell periods in such zones. Finally, there are difficulties in providing a seal for the filters, constructed as inserts in a slide, with respect thereto. Sealing is usually achieved by thrusting the filter insert against a seal surface by means of the pressure difference which occurs in operation; however, experience has shown that this cannot provide complete sealing, more particularly because the filter inserts must be inserted with some clearance into the slide in order to meet the demand for easy and rapid exchanging of the filters. The resultant gaps either cause undesirable bypassing of the filter or they form dead spaces in which molten plastics remains for a prolonged time and is thus thermally damaged. Changing the filter in known devices also causes relatively large amounts of air to be introduced into the plastics flow upstream and downstream of the filter.